Operation Masher, also known as Operation White Wing, (24 January—6 March 1966) was the largest search and destroy mission that had been carried out in the Vietnam War up until that time. It was a combined mission of the United States Army, Army of the Republic of Vietnam (ARVN), and Republic of Korea Army (ROK) in Bình Định Province on the central coast of South Vietnam. The People's Army of Vietnam (PAVN) 3rd Division, made up of two regiments of North Vietnamese regulars and one regiment of main force Viet Cong (VC) guerrillas, controlled much of the land and many of the people of Bình Định Province, which had a total population of about 800,000. A CIA report in 1965 said that Binh Dinh was "just about lost" to the communists.

The name "Operation Masher" was changed to "Operation White Wing", because President Lyndon Johnson wanted the name changed to one that sounded more benign. Adjacent to the operational area of Masher/White Wing in Quang Ngai province the U.S. and South Vietnamese Marine Corps carried out a complementary mission called Operation Double Eagle.

The 1st Cavalry Division (Airmobile) was the principal U.S. ground force involved in Operation Masher and that operation was marked as a success by its commanders. Claims are made that the PAVN 3rd Division had been dealt a hard blow, but intelligence reports indicated that a week after the withdrawal of the 1st Cavalry PAVN soldiers were returning to take control of the area where Operation Masher had taken place. Most of the PAVN/VC had slipped away prior to or during the operation, and discrepancy between weapons recovered and body count led to criticisms of the operation.

Allegations that there were a reported six civilian casualties for every reported PAVN/VC casualty during the Fulbright Hearings prompted growing criticism of US conduct of the war and contributed to greater public dissension at home. During Operation Masher, the ROK Capital Division were alleged to have committed the Bình An/Tây Vinh massacre between 12 February and 17 March 1966, in which over 1,000 civilians were allegedly killed. The operation would create almost 125,000 homeless people in this province, and the PAVN/VC forces would reappear just months after the US had conducted the operation.

Bình Định Province was a traditional communist and VC stronghold. Binh Dinh consisted of a narrow, heavily cultivated coastal plain with river valleys separated by ridges and low mountains reaching into the interior. The main effort of the campaign in Binh Dinh would come on the Bồng Sơn Plain and in the mountains and valleys that bordered it. The plain, a narrow strip of land starting just north of the town of Bồng Sơn, ran northward along the coast into I Corps. Rarely more than 25 km wide, it consisted of a series of small deltas, which often backed into gently rolling terraces some 30-90m in height, and, at irregular intervals, of a number of mountainous spurs from the highlands. These spurs created narrow river valleys with steep ridges that frequently provided hideouts for PAVN/VC units or housed PAVN/VC command, control and logistical centers. The plain itself was bisected by the east-west Lai Giang River, which was in turn fed by two others, the An Lao, flowing from the northwest and the Kim Son, flowing from the southwest. These two rivers formed isolated but fertile valleys west of the coastal plain. The climate in the region was governed by the northeast monsoon. The heaviest rains had usually ended by December, but a light steady drizzle, which the French had called crachin weather and occasional torrential downpours could be expected to occur through March. These weather systems would at times limit the availability of air support.

The vital artery of Highway 1 ran north and south ran through Binh Dinh. The area of Operation Masher was about 30 miles (48 km) north to south and reached a maximum of 30 miles (48 km) inland from the South China Sea. The U.S. Marine's Operation Double Eagle extended northward from Masher and the ROK's Operation Flying Tiger extended southward. South Vietnamese forces participated in all three operations.

The First Cavalry Division (Airmobile) was selected by U.S. Commander William Westmoreland to carry out the operation. The 1st Cavalry had borne the brunt of the combat during the Siege of Plei Me and the Battle of Ia Drang in October and November 1965, and some battalions of the 1st Cavalry had sustained heavy casualties. More than 5,000 soldiers in the division were recent arrivals in Vietnam with little combat experience. The South Vietnamese 22nd Division stationed in Binh Dinh had also suffered heavy casualties in recent fighting and was on the defensive.

The opposition to the American and South Vietnamese units participating in Operation Masher/White Wing was the PAVN 3rd Division consisting of approximately 6,000 soldiers in two regiments of PAVN regulars who had a recently infiltrated into South Vietnam via the Ho Chi Minh Trail and one regiment of VC guerrillas who had been fighting the South Vietnamese government since 1962. The majority of the population of Binh Dinh was believed to be supportive of the VC.